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    Fix #13777 by improving the underdetermined CUSK error message · ac91d073
    Ryan Scott authored
    The error message that GHC emits from underdetermined CUSKs
    is rather poor, since:
    
    1. It may print an empty list of user-written variables if there
        are none in the declaration.
    2. It may not mention any `forall`-bound, underdetermined
        variables in the result kind.
    
    To resolve these issues, this patch:
    
    1. Doesn't bother printing a herald about user-written
        variables if there are none.
    2. Prints the result kind to advertise any
        underdetermination it may exhibit.
    
    Test Plan: make test TEST=T13777
    
    Reviewers: goldfire, bgamari
    
    Reviewed By: goldfire
    
    Subscribers: rwbarton, thomie, carter
    
    GHC Trac Issues: #13777
    
    Differential Revision: https://phabricator.haskell.org/D4771
    ac91d073